Carter AFB tuning tip - Auxiliary airvalves One of the least understood issues of tuning the Carter AFB is the importance of the auxiliary airvalve. Carter supplied many different airvalves in their various AFB carburetors. Since most do not have a ready supply of airvalves, and they are not readily available, the easiest way to tune this variable is START WITH THE PROPER CARBURETOR FOR THE APPLICATION! The auxiliary airvalve controls the timing of when the air starts to flow in the secondary circuit. The fuel will have already begun to flow through the secondary starter circuit. The airvalve consists of a shaft with two valves, and two offset weights. Since Carter used a variety of weights, most enthusiasts think that the weight is the tunable variable AND NEGLECT TO LOOK AT THE ATTACK ANGLE OF THE VALVES. Yes, the weights ARE important, but the attack angle is easily as important, if not more so. We have had many carburetors come to us with weights ground almost to nothing, or chunks of lead screwed onto the weights when the angle of the valves was incorrect for the application. For STREET vehicles (you know, no 9000 RPM start from a stop sign, just spirited driving) GENERALLY, "torquer" engines like a small attack angle; and "screamer" engines like a large attack angle. Sometime take a look at the angle on an original AFB for a Pontiac, Buick, Chrysler, etc., and then compare it with the angle on the aftermarket AFB's. Using the large angle as found on the aftermarket (SBC) carburetors on the torquer type engine causes the airvalve to whip open too soon, which may cause a hesitation or bog when the secondary begins to open. Conversely, trying to use one of the Pontiac valves on a SBC will cause the engine to starve for air, and lose power as the secondary begins to open. Again, since few enthusiasts have a large selection of the various airvalves, the easiest method of tuning is to start with the correct carburetor. Carter did not build 505 different AFB's just to remove the R & D money from the profits of the shareholders. One size or type does NOT work for all applications. Jon.
My aftermarket 750 Carter AFB top (Federal Mogul made by Weber) needed a little dressing on the front of the air valve weights, about 1/16" and a 1/8" hole drilled in each side at the front of the valve to work right on an 800 Edelbrock AVS. I also had to swap the rear boosters because there was a fuel pullover passege in the AFB and I had to pull the skinny brass tubes out. The 750 has the 800 AVS top on it and both are perfect. On the 750 AFB top airvalve there are two little rises slightly cut in for those pull over feeds. It comes in like a freak at any speed you can't bog it, and it will stay wide open as long as you hold the throttle down. The AFB topped 800 is on a .020 over 390 Ford with 228°@ .050 cam and headers. The AVS top on the 750 is on a 352 with headers. I love my Holleys, but those two Carters/Eldelbrocks do pretty good after a little tuning. That is great info there on the air valve leverage, way overlooked, and I have used it to my advantage a few times. Have you ever worked on a Buick Q-Jet with the pullover enrichment feeds on the primary? I had one that I finally wore out that kept alot of guys scratching their heads on how it ran like that. Drilling the secondary feed tubes is tough, and it definitely liked a Holley Blue with 1/2" line, you could suck the bowl dry in third gear unless you had the volume
I haven't played with one in years, thanks for posting this. Regarding attack angle: what's the variable, the shape of the blade, position of the shaft, "at rest" stop?
The Jalopy Journal
